Major mass extinctions occur at both the beginning and end of the Triassic. The Triassic was initiated by the Permian-Triassic mass extinction, the extinction of more than three-quarters of all animal species living at the end of the Permian.
At the beginning of the Triassic, biodiversity was extremely low, which, however, gave new species the opportunity to develop.
This is how dinosaurs, pterosaurs (which could fly) and (small) mammals emerged during that period.
Although some dinosaurs grew to around six meters in length during the Triassic, the truly larger dinosaurs did not appear until the Jurassic.